Targeting of bacteria to solid tumors has been demonstrated for several bacterial species including Bifidobacterium spp. (Kimura et al. (1980) Cancer Res. 40:2061-8), Listeria monocytogenes (Liu et al. (2007) Cancer Res. 67:429-32; Riedel et al. (2007) Appl Environ Microbiol. 73:3091-4), Clostridium spp. Bettegowda et al. (2005) Proc. Natl. Acad. Sci. U.S.A. 102:1145-50), Salmonella spp. (Toso et al. (2002) J. Clin. Oncol. 20:142-52), Shigella flexneri (Stritzker et al. (2007) Int. J. Med Microbiol. 297:51-62) Vibrio cholerae (Yu et al. (2004) Nat. Biotechnol. 22:313-20) and E. coli (Stritzker et al. (2007) Int. J. Med. Microbiol. 297:51-62). The use of certain bacteria for therapy and diagnosis of tumors, however, is often impeded by risks of opportunistic infections and low tumor colonization rates. For example, in clinical trials of Salmonella VNP20009, a low percentage of tumor-targeting efficacy was observed (Toso et al. (2002) J. Clin. Oncol. 20:142-52). Thus, for bacterial therapy and diagnosis of tumors, there is a need for methods that employ bacteria that are non-toxic to the patient, can be administered at low doses and show a high degree of preferential accumulation in tumors versus normal tissues.
In addition, biopsy is currently the only clinical method available for determining the presence of bacteria in the tumor. A majority of studies on bacterial tumor colonization in tumor-bearing mice have used luciferase- and/or fluorescence-(GFP) imaging for bacterial detection. However, current optical imaging modalities using fluorescent proteins or luciferases are restricted to small animals and cannot be readily translated to patient studies. Hence, there is a need for methods for non-invasive imaging of bacteria-colonized tumors to accurately detect the presence of bacteria in tumors without excision of the respective tissue.
Overexpression of HSV-thymidine kinase in tumors has been used to localize radiolabeled tracers to tumors for tumor imaging by positron emission tomography (PET) (Serganova et al. (2004) Cancer Res. 64:6101-8; Tjuvajev et al. (1998) Cancer Res. 58:4333-41; Tjuvajev et al. (2002) J. Nucl. Med. 43:1072-83; Jacobs et al. (2001) Lancet 358:727-9). Salmonella bacteria that overexpress HSV-thymidine kinase, Salmonella VNP20009, have been employed to image C-38 colon carcinoma and B16-F10 murine melanoma by PET using the radiolabeled tracer [124I]-FIAU (Soghomonyan et al. (2005) Cancer Gene Ther. 12:101-8). However, gene expression systems that are designed to overexpress proteins can be highly variable, leading to varying amounts of gene expression at the site of accumulation. Such systems are not quantitative and cannot be used to accurately measure the accumulation of the administered bacteria within a tumor over the course of treatment, diagnosis or monitoring of a tumor. Hence, there is a need for methods to accurately measure the amount of bacterial accumulation in a tumor in order to use the bacteria for diagnostic methods, such as monitoring therapeutic treatment and/or optimizing therapeutic regimens and with decreased reliance on foreign protein expression.